1st-cycle Mew Gull from Mile Square Park, 1/1/12
The Desert Center Mew Gull, 11/16/14
Ah, North America. Like Varied Thrush last week, the Pacific Northwest seems to be the core of the reports. Records away from the Pacific Coast are scattered widely across the whole country.
Also, Mew Gull reaches the southern end of its (wintering) range in southern California (though, note the records from northern Baja and the Gulf of California--there would be more records if more people birded there. Perhaps it is not judicious to claim southern California represents the "southern edge" of ranges based on eBird maps. Hmm.)
Zooming in to California, we see that they stick pretty close to the coast, with the Salton Sea and the Central Valley the only places they wander far inland consistently. Closer to the coast (e.g., Los Angeles County, Orange County, western Riverside County) it consistently wanders inland a short distance, often frequenting small urban lakes.